A common problem that is encountered in vehicles such as fixed wing or rotary wing aircraft is unwanted vibration forces that are induced into the vehicle frame by imbalance forces of rotating machines. One method that has been developed to address these unwanted vibrations is to generate controlled vibration cancellation forces by spinning motors equipped with specified imbalance masses. These motors, or rotary actuators, are controlled to operate synchronously with the vibration frequencies to produce imbalance reaction forces such that a desired vibration cancellation effect is produced. The rotary actuators can have safety concerns because it is possible for faults in the electronic control system to cause the cancellation forces to shake the vehicle structure at an unintended frequency or incorrect magnitude or phase. Such errant rotating forces can increase vibration (e.g., by exciting resonances in the vehicle) and result in out of limits, unsafe operating conditions, structural damage or even catastrophic loss.
The potential of the electronic control system to command the rotary actuators to produce forces at prohibited (undesired) magnitude, phase or frequencies and the critical safety implications of the vehicle structure being excited at those frequencies combine to impose a flight critical, or Level A, classification of the control system. The flight critical classification mandates the need for an independent monitoring system with the capability to detect the undesired force magnitude, phase, speed and/or vibration condition and the authority to disable the rotating forces if the condition does not clear within some period of time or number of cycles. A Level C condition constitutes a significant reduction in safety margins or functional capabilities, a significant increase in crew workload or conditions impairing crew efficiency, and/or some discomfort to the occupants.